1. Field of the Invention
The present invention relates to an image forming apparatus such as an electronic photographing apparatus, and to a developing apparatus provided in the image forming apparatus.
2. Description of the Related Art
In general, an image forming apparatus such as a copying machine comprises a photosensitive body on which an electrostatic latent image corresponding to a desired image is formed; a charging apparatus for charging the surface of the photosensitive body at a predetermined potential; an exposing apparatus for exposing the photosensitive body and forming an electrostatic latent image thereon; a developing apparatus for developing the electrostatic latent image and forming a visible image; a sheet feed unit for feeding a recording paper sheet to the vicinity of the photosensitive body; a transfer apparatus for transferring the developed visible image onto the fed paper sheet; and a fixing apparatus for fixing the visible image transferred onto the recording paper sheet.
In the image forming apparatus, at first, the surface of the photosensitive body is uniformly charged by the charging apparatus at the time of image formation. At this time, the surface potential of the photosensitive body is charged at, e.g. -500 V. Next, the photosensitive body is exposed by the exposing apparatus on the basis of image data. At this time, the surface potential of the exposed region, i.e. the region with the electrostatic latent image, is, for example, -50 V, and the surface potential of the non-exposed region is kept at -500 V. Toner charged at, e.g. -200 V is adsorbed on the electrostatic latent image on the photosensitive body by the developing apparatus and the electrostatic latent image is visualized. In other words, the electrostatic latent image is developed and a toner image is formed. The toner image is transferred onto the recording paper sheet fed from the sheet feed unit by the transfer apparatus. The transferred toner image is fixed by the fixing apparatus.
An example of the developing apparatus provided in the image forming apparatus is a developing apparatus 100 using non-magnetic one-component toner. As is shown in FIG. 1, the developing apparatus 100 comprises a developing roller 102, a supply roller 104, a layer thickness limiting member 106, a stirring paddle 108 and a body casing 110 containing these structural elements 102, 104, 106 and 108. The body casing 110 has a toner container 112 for containing toner. A front opening of the body casing 110 is situated to face the photosensitive body 200. In addition, the developing roller 102 is provided at the front opening of the body casing 110 such that the roller 102 is rotatable in a predetermined direction.
The supply roller 104 is provided such that the roller 104 can rotate while closely contacting the developing roller 102 and toner T can be supplied from the toner container 112 to the outer peripheral surface of the developing roller 102. Specifically, the supply roller 104 extends in a direction perpendicular to the surface of the sheet of FIG. 1 and comprises a shaft member 104s rotated in the same direction as the developing roller 102 by an external driving mechanism and a cylindrical roller member 104r formed of an elastic material such as sponge and fitted on the shaft member 104s.
The layer thickness limiting member 106 has a thin plate shape and a proximal end portion thereof is fixed to a front portion of the body casing 110. A distal end portion of the layer thickness limiting member 106 is provided with a toner layer limiting member 106a formed of an elastic material. The toner layer limiting member 106a is put in pressure contact on the outer peripheral surface of the developing roller 102. The toner T supplied on the outer peripheral surface of the developing roller 102 by means of the supply roller 104 is electrified by the toner layer limiting member 106a, and a thin toner layer of predetermined thickness can be formed.
The stirring paddle 108 is rotatably provided within the toner container 112, thereby to prevent cohesion of toner and supply toner to the supply roller 104.
In the developing apparatus having the above structure, when an image is to be formed, a toner layer formed on the outer peripheral surface 102s of the developing roller 102 comes into contact with the photosensitive body 200, and the toner is supplied to the region of the electrostatic latent image (the exposed region) of the photosensitive body 200. The toner not supplied on the photosensitive body 200 and left on the developing roller 102 passes through a recovery sheet 114 and is recovered into the body casing 110.
Jap. Pat. Appln. KOKOKU Publication No. 5-10672 discloses toner supply device using no supply roller. According to this toner supply device, toner is pushed and supplied into a gap between a toner reservoir, which faces a developing roller in a non-contact manner, and the developing roller by means of a paddle for drawing up toner from a toner container.
In the method using the supply roller, a mechanism and energy (torque) for rotating the supply roller, in addition to the developing roller and stirring paddle, are required, resulting in a higher cost. Since the miniaturization of the supply roller itself is limited, it is difficult to reduce the size of the apparatus and simplifying the structure of the apparatus.
On the other hand, in the method using no supply roller, the above-mentioned problems are less serious. However, it is difficult to stably supply toner to the region closest to the developing roller. In addition, it is difficult to stably apply pressure to the toner in order to adhere the toner to the developing roller.
Specifically, in the developing apparatus described in Jap. Pat. KOKOKU Publication No. 5-10672, toner is supplied from the toner reservoir provided near the lower part of the developing roller, the amount of toner supplied to the region closest to the developing roller tends to be deficient. Moreover, a blur of images and a decrease in toner density may occur due to deficient pressure on the toner. Besides, the toner may be excessively pushed and supplied by the toner draw-up member, resulting in cohesion of toner and deficient supply of toner.
In a developing apparatus described in Jap. Pat. KOKOKU Publication No. 2-26228, a toner container is situated above a developing roller, and a toner layer thickness limiting blade for supplying toner is provided in contact with or with a gap from the developing roller near the outlet of the toner container.
In this developer apparatus, however, the weight of toner itself acts on the toner in the toner container and the toner tends to cohere due to excessive pressure. As a result, the toner does not uniformly adhere to the developing roller, and parts of a printed image may miss in white stripes.